Gas references are widely used in numerous applications involving optical signals. Indeed, light can interact in various fashions with gases, depending on the type of light signal, the type of gases and the optical configuration used.
One application of gas cells for using with optical signals is the absolute referencing of the optical frequency of a light signal from a device such as a laser. Indeed, if the gas has a well known light absorption characteristics with optical frequency, the gas can be used as an absolute reference filter to accurately determine the light frequency and eventually control it.
There are a number of applications that use laser sources and wherein absolute, calibration-free knowledge of the frequency of the laser is very desirable for better performance and long life. For instance, wavelength division multiplexed (WDM) communication system offer a high data transmission capacity by allowing multiple laser sources to transmit many high-speed data channels simultaneously into a single fiber, where each channel is transmitted at a unique optical frequency (or wavelength). In order to standardize the frequencies of the channels across telecommunication systems, the industry has adopted a standard which specify that the nominal optical frequency of every channel should be at an integer multiple of 100 GHz, 50 GHz, 25 GHz or even smaller spacings with a typical accuracy within 2.5 GHz, 1.25 GHz or even better as the grid density increases. Semiconductor lasers currently used in telecommunication systems do not intrinsically generate frequencies accurate or stable enough to be used in such dense frequency grids because current fabrication technologies do not permit to know with sufficient accuracy the nominal frequency of the lasers, and the frequency of the laser varies significantly with operating conditions and environmental factors such as temperature and ageing. For these reasons there is a need to stabilize the frequency of semiconductors to a predetermined value with a sufficient accuracy by using an optical reference element to compare the frequency of the laser with the predetermined value and generate an error signal which is fed back to the laser to correct its frequency. Different optical references have been used on the past to stabilize semiconductor lasers. Some atomic or molecular gases, for instance, exhibit absorption lines in the optical frequency range of telecommunication networks. The frequency of these absorption lines is determined by quantum mechanical laws and is generally extremely precise and stable with respect to environmental factors. Furthermore, the width of the absorption lines is very narrow, which allows for very sensitive frequency drift detection. Once properly frequency-locked to an absorption line, a laser can display frequency accuracy and stability orders of magnitude better than is required for current telecommunications systems. Gas cells are therefore a reference of choice for those high frequency accuracy applications.
Another application where gas cells are used in combination with an interrogating optical beam is spectroscopy. Gas analysis systems typically use a gas cells to hold a gas sample whose composition is to be determined. A broadband light source is used to generate a light beam that is passed through the gas in the cell, and a spectrometer measure the resulting light spectrum. The dips in the measured spectrum shows at which frequencies the gas has absorbed the light and with which intensity. The composition of the gas can then be deduced by processing the absorption spectrum. Such application may be used, for example, for sensing of foreign substances.
Other type of spectrometers or gas analysis devices may use narrowband light sources to interrogate a gas and determine the concentration of specific substances. These may use narrowband sources such as a tunable or fixed-frequency lasers. These instruments work by passing the laser light through the gas inside an optically transparent gas cell and by measuring its absorption at one or many frequencies. The composition of the gas can then be deduced by processing the absorption values. Such a system can be as simple as a laser probing one optical frequency only which is known to be absorbed when a gas is present. Such a calibration technique us described in U.S. Pat. No. 5,780,843.
Spectrometers such as the ones described before may require calibration to ensure the measured frequencies and absorption level are accurate. To achieve high accuracy, samples of known gases are stored in sealed cells as references for spectroscopic quantities, and are optically interrogated by the spectrometer to measure their spectrum and recalibrate the spectrometer amplitude and frequency scale.
Optical measurements instruments such as Optical Spectrum Analyzers (OSA), performance monitoring systems and other spectrum analysis devices can also advantageously use gas cells as calibration devices. Many commercial OSA now possess an internal gas cell containing a well-known gas. The light from a broadband source such as a LED is passed through the cell and its spectrum is recorded by the instrument. The measured spectrum is then analyzed to find frequency calibration points from known absorption lines of the gas and can therefore compensate frequency offsets and drifts of the instruments. Acetylene gas cells are commonly used as references at wavelength around 1530 nm. The use of a gas cell to calibrate an OSA is described in U.S. Pat. No. 6,421,120.
Heterodyne-based optical spectrum analyzers are another kind of spectral analysis devices. These use the heterodyning between a local tunable laser source and the user optical signal to measure the power spectrum the user signal with a very high resolution. An internal gas cell can also be used to frequency-calibrate the local tunable laser by measuring and analyzing the known absorption lines observed when the laser is tuned.
Different types of gas cells exist on the market. Gas cells made entirely of glass are very common. Since they are made entirely of glass, the two ends of the cells are made of transparent materials and allow for a light beam to pass through them an be absorbed by the gas inside. Glass cell present several drawbacks, however. First, conventional glass cells are not very compliant to mechanical stresses and are fragile. Consequently, they are less useful in aerospace applications or in industrial environment or portable equipment where shocks and prolonged vibrations can jeopardize the cell structure and its hermeticity. Second, glass gas cells involve high temperatures for sealing. Indeed, such cells typically possess a glass filling tube attached to the side of the cell. Typical assembly techniques consist in connecting a vacuum pump to the filling tube, by emptying the cell, and by introducing the desired gas. Then a portion of the filling tube is heated until the glass softens and part of the tube collapses upon itself under the internal vacuum pull. This, effectively seals the cell. The high temperatures required to soften the glass may cause the gas inside the cell to react and loose its chemical properties. Special type of glasses may be used to allow a lower melting point, but the required temperatures are still relatively high. Furthermore, the sealing process described above is often done manually by people skilled in fused glass manipulation. This sealing process is however more difficult or expensive to implement in fabrication chains with reliable seal quality, and the geometry of the sealed filling tube is hard to control accurately. High quality glass cells are therefore expensive to fabricate.
In order to avoid using high temperatures for sealing the glass gas cell, organic sealants such as glue or epoxy could be used to seal the glass filling tube. Such materials, however, are subject to degradation with time and may be affected by exposure to moisture or other chemicals. Organic sealants generally let gases slowly diffuse through them and are not considered hermetic. Telecommunication components which are subject to the harsh Telcordia reliability requirements generally keep all organic glues within a moisture-free hermetic package which is sealed by soldering, brazing or welding in order to prevent degradation.
Another disadvantage of glass cell is that glass is not a convenient material to shape or modify. It is harder to shape the glass cell into the desired format, especially if the interior of the cell is kept at very low pressures compared to atmospheric pressures.
Thus, in order to avoid the limitations of glass cells, many applications rely on using metallic gas cells to contain a gas that can be interrogated by a light beam. Such cells are potentially more robust and flexible than glass cells. However, these cells must still possess optical windows to allow the light to pass through the gas.
There is in the art a few kind of metallic cells that can be used with optical signals. Examples of those can be found in applications such as optically-pumped rubidium or cesium clocks. In these applications, a light beam is sent inside the metallic cavity to excite a gas, which then resonates at microwave frequencies. The metal cavity also serves as a microwave resonator and allow the atoms to continue resonating at their natural frequency which is very stable and precise. Some of the microwave signal is extracted from the cavity to serve as a accurate frequency standard from which precise time measurement systems can be built. U.S. Pat. No. 5,327,105 describe such a gas cell used for a miniaturized atomic frequency standard.
Industrial system also use metal enclosures to contain a gas while allowing to probe it with a light source. These systems are often made of stainless steel tubes bolted together in various configurations with flange joints and hermeticity rings made of rubber or soft metals. Optical windows made of glass or other transparent materials are secured and sealed onto the main assembly through additional flanges and bolts. To use the system, gases from an industrial process are made to flow inside the metal tubing, and light is passed through the gas through the windows. A spectrometer or other instrument can then be used to analyze that light and determine the process gas composition.
Optical modules used in telecommunications are other examples of metallic enclosures which allow light in or out and are hermetic. A typical laser module, for example, consists in a square Butterfly-type package in which all the optical components, including the laser source, are glued or otherwise attached. A metallized optical fiber passes through an opening on one wall and is hermetically soldered to that wall. Once the components are in place, a lid is placed on the package and is hermetically soldered by welding, soldering or brazing, generally in an inert moisture-free gas such as nitrogen. This results in a metallic cavity filled with nitrogen with an optical output port (the fiber). Another example of sealed optical component are photodetectors, which may be packaged in a can-type package with an hermetic window. The photodetector chip is placed in the can, generally filled with an inert, moisture-free gas. Its surface can receive optical energy through the window placed at the other end of the can.
Optical components such as the ones described above are not designed to withstand very low vacuum levels, and generally make no provision for connecting a vacuum system as there is no filling tube or hole.
Some specialized opto-electronic components, such as photomultiplicator tubes or bolometers, are vacuum-tight components that have windows to allow light to enter the inside of an evacuated cavity. These devices are not designed to allow a light beam to interrogate a gas in order to use it as a optical reference.
One main disadvantage of the metallic gas cells described above is that these components are often manufactured from many components, v.i.z. side walls, windows, floor, sealed lid, etc. All these components need to be attached together hermetically, and a number of joints have to be sealed perfectly to maintain a good vacuum for long periods. In such set-ups, the total number of hermetic joints is not minimized, a situation which increases the risk of leaks and device failure. Having to assemble these components also make the device expensive to fabricate.
One improved method of fabrication for these metallic gas cells would be to use molded or pressed metal enclosures on which lids are hermetically attached. Although this would minimize the number of hermetic seals, the total length of the seals would still not be minimized.
Another disadvantage of previously described metallic gas cells is that the structure of these cells do not allow easy alignment of the input or output optical components. In many applications where gas cells are required, the light beam that enter the cell must keep a very accurate alignment to reach the other end of the cell at a very precise point and with a very accurate angle. The previously metallic gas cells do not have mechanical structure that allows to point a beam accurately enough to adequately hit a small surface detector, a fiber collimator, or any other target at the exterior of the cell.
As has been shown above, metallic gas cells do bring a number of advantages compared to glass cells, although a number of structural properties would need to be improved in order to provide cheap, reliable and very accurate gas cells. There are also a number of additional features that are lacking in gas cell currently available. Those are discussed thereafter.
There are applications where it is not sufficient to have only one gas reference and where multiple gas references may be needed, for example to be interrogated by a laser, spectrometer or OSA. For example, a widely tunable laser may use the absorption lines of a gas to calibrate its frequency tuning characteristics, but a single gas may not provide absorption lines over the whole frequency tuning range of the laser, which does not allow the laser to be fully calibrated over its full range. In some cases, it may be possible to mix within a single gas cell multiple gases having various absorption lines at various frequencies to provide the requires calibration points over the full tuning range of the laser or the instrument. However, some gases cannot be mixed because they react with one another, or the presence of other gases broadens or shifts the absorption lines, therefore reducing their accuracy. In such situations, multiple independent gas cells must be used, but each cell require their own hermetic input and output port (window, fiber collimator etc.). This increases the number of interfaces the light beam must cross while going through the cells. This brings the disadvantages of increased part count, increased cost, and increased optical losses. Current gas cells designs do not allow to efficiently cascade multiple gas cells while minimizing these drawbacks.
Another commonly found drawback with current gas cells is that they require an additional, independent fiber collimator in order to use them with an fibered input or output signal. Indeed, many applications use optical fiber to bring light into a gas cell. The light exiting the fiber has to be collimated in order to pass through the gas, therefore requiring a fiber collimator. This collimator, which consists in a fiber attached to a lens, is installed in front of the input window of the gas cell. This bring a number of disadvantages. First, the light exiting the fiber has to go through two optical components, that is, the lens and the cell window, before reaching the gas. This increases the number of optical components, the optical losses and the risk of parasitic optical resonances. Second, since the fiber collimator is mechanically independent from the gas cell, the alignment of the collimated beam into the cell may inadvertently change with external mechanical stresses. None of the existing cells integrate a collimator into the cell itself for eliminating components and providing a better performance and alignment stability. The same limitations exists if there is a need to collimate the beam exiting from the cell into a fiber.
Similar limitations have been found in applications which require the beam exiting a conventional gas cell to be sent onto a packaged photodetector. In these applications, the light beam must exit the gas through the cell window, will cross the photodetector window and will then hit the photodetector chip. The presence of two optical interfaces, of which at least one is redundant, also causes parasitic reflections and resonances and increase set-up cost.
Known in the art, there is U.S. Pat. No. 5,025,448 granted to SUDO. In this patent, SUDO describes a method and a gas cell for stabilizing frequency of semiconductor laser. The proposed cell has a reduced number of components by directly attaching lenses, bare fibers and photodetectors at the extremities of a cell (generally made of glass) in order to hermetically seal it. Although this method solves some of the drawbacks previously mentioned, this invention still presents several disadvantages. First, the input of the gas cell is not adapted for directly attaching an optical fiber to the cell and providing a collimated beam into the gas, which would be required if the light is to travel relatively long distances for detecting weak absorption lines of gases. In the proposed embodiment, the non collimated fiber is installed in a sealed tube passing through one end of the cell. The addition of the tube increases the number of orifices in the cell, the number of components, and also increases the risks of leaks.
Second, the output of the proposed cell is either a photodetector or a fiber obtained from a drawing rod of glass into a fiber. This last device does not offer the same flexibility and availability as a standard collimator made from a lens and an attached fiber. No solution is provided to provide a free-space collimated output beam.
Third, these cells do not allow to be used as modular units. They cannot be attached together to form multiple cavity cells or cannot be attached to a optoelectronic package.
Finally, this invention does not disclose how the optical components can be attached without using organic adhesives, nor how the gas cell, one filled, can be sealed without using the same organic materials or high temperature processes.
Also known in the art, is U.S. Pat. No. 4,119,363, which describes an hermetic metallic package that features an hermetic lens and fiber outputs. This invention provide very good hermeticity, and is intended to be filled with non-corrosive gas such as nitrogen. The invention, however, is constituted of many mechanical parts and seals that are not suited for very low cost gas cell fabrication.
U.S. Pat. Nos. 5,268,922 and 5,500,768 describe how laser sources or photodetectors can be packaged into an hermetic package having an optical output consisting of a window or a lens. These devices are not gas cells and have a large number of components, but illustrate the principle of using the lenses as components to seal the end of an hermetic cavity. U.S. Pat. No. 5,793,916 and U.S. patent application No. 2002/0118463 are other examples of specialized fibered devices that require hermetic seals but are still not dedicated to gas absorption measurements.
Therefore, there is a need for a new metallic gas cell that would overcome the drawbacks of the existing cells described above.
It would be advantageous to provide a method for manufacturing a metallic gas cell that possesses the minimum number of parts and has a low material cost.
It would be advantageous to provide a metallic gas cell that includes the minimum number of walls, a minimum number of joints and a minimum total length of hermetic joints in order to minimize the probability of a leak.
It would furthermore be advantageous to provide a metallic gas cell that does not possess end walls; the end optical components acting themselves as effective walls.
It would also be advantageous to provide a metallic gas cell wherein a limited number of components can be assembled in order to produce various cell configurations. This would reduce cost for stocking in high volume production while providing high product flexibility.
It would also be advantageous to provide a metallic gas cell having a reduced number of components by installing an optical fiber collimator or a collimating lens directly at the input of the gas cell, effectively sealing it.
It would also be advantageous to provide a metallic gas cell having a reduced number of components by installing a photodetector, a collimating lens or an optical fiber collimator directly at the output of the gas cell, effectively sealing it. This would eliminates the need for an output window, and would reduce the risk of parasitic resonances.
It would also be advantageous to provide a cell that can be sealed without using high temperatures.
It would also be advantageous to provide a metallic gas cell whose sealing process can be easily automated.
It would also be advantageous to provide a modular metallic gas cell which is attachable to another gas cell in order to provide multiple cavities gas cells while minimizing the number of optical components, therefore reducing the optical losses, parasitic reflections, resonances, and cost to a minimum.
It would also be advantageous to provide a metallic gas cell able to be attached to other types of optical modules, such as, for example, butterfly packages.